Century Base changes over the years

[Buff Mair]

I’m new to the forum and have already learned a great deal about vintage lava lamps. Can someone please tell me what year did the base of Century lava lamps change from having one screw to two screws? Also, when did the hold pattern change from a tighter arrangement to further apart? Thanks

[Buff Mair]

Just realized I made a typo- I meant “hole” pattern, not “hold”.